Selective crystallization of four bis(phthalocyaninato)lanthanoid(iii) polymorphs

Abstract

Bis(phthalocyaninato)lanthanoid(III) (LnPc₂) complexes have attracted significant attention for their exceptional optical, electronic and magnetic properties. Crystallization of these compounds usually requires cumbersome methods such as sublimation and electrocrystallization, which is a significant limitation to both structural determinations and the preparation of high purity materials at scale. We report here the selective crystallization of four polymorphs of LnPc₂ obtained exclusively by the slow evaporation of saturated solutions. The obtained phase depends on the initial oxidation state of the LnPc₂ molecule and the choice of solvent. Single-crystal X-ray diffraction studies were used to determine 14 new structures including Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Er and Yb, as well as correct previous mis-identifications from the literature. We provide a detailed comparison of molecular structure and crystal packing in all LnPc₂ polymorphs. The primary feature in all phases is columnar stacking based on parallel π–π interactions, with a variety of slip angles within those stacks as well as secondary interactions between them. Chemical redox and acid–base titrations, performed on re-dissolved crystals demonstrate that LnPc₂⁺ and LnPc₂⁻ are easily obtained through weak oxidizing and reducing agents, respectively. Additionally, we show that the protonated form of the NdPc₂⁻ complex has a nearly identical UV-vis spectrum to that of neutral NdPc₂, explaining some of the confusion over chemical composition in previously published literature.